The present invention relates to a process for the production of polysulphide silanes using an inorganic polysulphide produced in a novel manner. In another aspect, the present invention relates to a method of producing inorganic polysulphides.
The use of bis(triethoxysilylpropyl)tetrasulphane combined with silicas having an elevated surface area in order to improve the properties of automotive tires, such as abrasion, wet skidding resistance and rolling resistance is prior art which has been documented in numerous patent applications. The difunctional silane here in the above compound acts as a bridge between the hydrophilic inorganic filler, silica, and the hydrophobic organic polymer, wherein a strong covalent bond is formed between the filler and the polymer.
The silylalkylpolysulphanes are essentially produced by nucleophilic substitution on chloroalkylsilanes with anionic polysulphides produced by various methods. Such a process was first described in German patent DE 2141159, but this document gives no information relating to the production of the nucleophilic polysulphide. However, production of the nucleophilic polysulphide is the critical part of the process which gives rise to silylalkylpolysulphanes.
Some patents have described the use of hydrogen sulphides to produce the precursor polysulphide. The German patent DE 2542534 accordingly proposes a method for the production of bis(silylalkyl)polysulphanes in which the polysulphide is formed in situ from hydrogen sulphide and sulphur in alcohol. Hydrogen sulphide is liberated in this process and must be disposed of by suitable measures. This disposal entails additional, economically undesirable, capital costs for an industrial plant. While liberation of hydrogen sulphide may indeed be suppressed by adding alkoxides (German patents DE 2712866 and German published application DE-OS 3311340), this approach is disadvantageous for technical and economic reasons.
Firstly, the anhydrous hydrogen sulphide required for performance of this process must be obtained in a preceding process stage, for example by a reaction between alkoxides and hydrogen sulphide (c.f. U.S. Pat. No. 5,399,739). Hydrogen sulphide, being a highly toxic gas, constitutes a considerable safety hazard, which must be minimized at some cost by suitable technical measures. Furthermore, most alkoxides have only limited storage stability, so entailing continuous in situ production. This additional starting product thus considerably increases the production costs of the desired bis(silylalkyl)polysulphanes.
One possibility for producing the nucleophilic polysulphide is to react sulphide with sulphur. U.S. Pat. Nos. 5,405,985 and 5,468,893 and application EP-A 694552 propose processes which obtain the corresponding polysulphide in an aqueous solution from sulphides and sulphur and react it with haloalkylsilanes in a two-phase system by phase transfer catalysis to yield polysulphanes. In this method, the phase transfer catalyst remains in the product thereby exerting an as yet unexplained influence on the applicational properties of the bis(silylalkyl)polysulphanes. Furthermore, due to the known susceptibility to hydrolysis of the alkoxysilanes used, the products are distinguished, as may be expected, by poor storage stability, thereby further restricting the customer's processing options. For these reasons, two-phase synthesis with a polysulphide produced in an aqueous solution is clearly not an acceptable approach.
There are thus reasons for using anhydrous polysulphide for the reaction between inorganic polysulphide and alkoxysilane. The problem then arises that anhydrous sulphides are not commercially available.
Japanese published patent application JP 7-228588 proposes two processes for the production of a polysulphide to solve this problem. In the first process, a hydrous sodium sulphide is dissolved in a solvent mixture and the water of crystallization present therein is removed by azeotropic distillation of the solvent. In the second method, once drying has been performed under a vacuum, the sodium sulphide is reacted with sulphur in anhydrous ethanol.
Very long reaction times of 5 hours are required for synthesis of polysulphides from sulphides and sulphur in boiling alcohol. This is substantially because sodium sulphide and sulphur have only very limited solubility in the solvents used. Diffusion of the two reactants towards each other thus becomes a determining factor for the rate of reaction. Were the proposed process scaled up to the industrial scale, it is to be suspected that this reaction time would be still longer.
However, with regard to increasing the space/time yield and thus for ensuring better utilization of capital investment, it is necessary to shorten the reaction time in order to achieve an economically viable reaction. The sodium polysulphide is furthermore obtained in the form of alcoholic solutions/dispersions. This proves to be very disadvantageous if further processing and production are not performed on the same site.
It is therefore an object of the present invention to avoid the prior art problems and to provide a process which yields a dehydrated, solid product with shorter reaction times.